This invention relates to tube cutting devices and techniques and, more particularly, to a method and apparatus for separating flat heat exchanger tubing into predetermined lengths having substantially burr-free parted ends.
Conventional heat exchanger tubing is fabricated from cylindrical, thin-walled copper pipe, which is bent into a serpentine pattern and provided with heat exchanging fins. It has long been recognized that care must be taken in cutting the tubing to minimize the formation of radially inwardly projecting burrs which are formed by the cutting tools. Typically, the tubing is cut by orbiting a cutting tool about the circumference of the tubing while moving the cutter inwardly.
Various mechanisms have been proposed to minimize the formation of burrs on such tubing. One such device is disclosed in U.S. Pat. No. 3,568,488 to Franks. According to the Franks patent, a tube is tightly gripped and placed under tension, and while the tube is tensioned, a cutting tool makes an annular cut in the periphery of the tube which penetrates into the wall of the tube. With the tension applied to the tube, the tensile strength of the tube wall not yet severed is exceeded, and the tube breaks. Consequently, the tube material is not cut entirely through, but partially cut and partially pulled apart at the cutting point. Therefore, the inwardly extending burr is minimized by the extruding caused by the pulling apart action. Other examples of tube cutting techniques may be found in U.S. Pat. Nos. 4,235,137; 4,112,294; and 3,692,219.
A recent development in heat exchanged tubing involves the provision of extruded flat tubing made from aluminum. The flat tubing has flat, parallel faces and rounded ends to define a flat, narrow interior channel. The interior channel is separated into a plurality of axially extending internal chambers defined by parallel walls extending perpendicularly between the flat walls of the tube. This arrangement provides a large surface area to the liquid flowing through the tube. Since the walls defining the internal chambers are so closely spaced, it is imperative that the severed ends of the tubing be substantially burr-free so that the burrs do not block the passages and do not interfere with an edge-to-edge tube joining operation or other operation involving the connection of the tube to a fluid source.
An improved tube parting technique has been developed to accommodate the dimensions and shape of the flat tubing which separates the tubing in such a manner that the separated end of the tubing is substantially burr-free. This improvement is set forth in U.S. Pat. No. 5,143,268, granted to Steven L. Stroup, Jr. According to the Stroup, Jr. patent, a length of flat metal tubing is clamped at spaced apart locations by gripping devices. Circular cutting or scoring blades deeply score lines of equal depth on opposite faces of the tubing with each line being in a plane normal to a longitudinal axis of the tubing. The scored lines do not penetrate the entire thickness of the flat tubing walls. A high impact force is imparted along the longitudinal axis of the tubing by sharply striking one of the gripping devices with a high impact force. The magnitude of the force is sufficient to separate the flat, parallel faces of the tubing at the score line and to separate the internal parallel walls and the rounded ends. The sudden impact fractures the tubing along the scored line without any substantial elongation or cold flow of the metal to thereby eliminate, or substantially reduce, burrs and necking down of the tubing.
Another technique for separating thin-walled multi-port extrusion tubing is set forth in U.S. Pat. No. 5,133,492, granted to Wohrstein et al. According to the Wohrstein et al. patent, a length of tubing is clamped between upper and lower clamping members. The upper and lower clamping members include straight edged knife blades which are forced into the upper and lower walls of the tubing to a depth which is slightly less than the thickness of the upper and lower walls. The tubing is parted by drawing a first portion of the partially severed tubing away from a second portion of the tubing in a direction perpendicular to the grooves formed by the cutting blades. According to the patentees, the elongation characteristics of the metal is such that the metal necks down into a bell shape to minimize interference with the cross-sectional opening of the tubing.
One problem associated with the technique described in the Stroup, Jr. patent is the effect of the upper and lower circular cutting or scoring blades on the cross-section of the tubing as the line is being scored. The circular cutting blade is rotatably mounted, and the effective cutting area comprises the arc of the circle defined by the cutting blade as it engages the tubing wall. The length of this arc is extremely small and, therefore, significant inward force is exerted on the tubing by the cutters. As a result, care must be taken to avoid total severance of the flat walls since such severance would result in inwardly directed burrs. On the other hand, even when the walls are not completely severed, the walls tend to be crushed to thereby reduce the effective cross-section of the tube passages.
Similarly, in accordance with the teachings of Wohrstein et al., all of the cutting force is directed perpendicular to the faces of the extrusion, and inward deformation of the tubing is substantially unavoidable. Furthermore, since one aspect of the Wohrstein et al. technique relies upon the cutting blades to restrain the tube during the drawing process, the blades must have a thickness which is sufficient to resist the shear stresses imposed on the blades by the drawing operation. Since the thickness of the blades must be increased to provide resistance to shear, the metal displaced by the blades must necessarily tend to be displaced into the tube opening.
Applicant's co-pending application Ser. No. 07/998,421 provides method and apparatus for separating flat aluminum heat exchanger tubing by applying clamping pressure to the tubing at spaced apart locations and through cutting opposite parallel flat walls of the tubing transversely to the longitudinal axis of the tubing. The cutting blades have straight edges which are inclined at an angle to the flat faces of the tubing and which have a leading edge spaced outwardly with respect to the outer faces of the tubing, and a trailing edge which is spaced slightly inwardly with respect to the inside face of the flat tubing wall. The angle defined by the cutting blade with respect to the flat face of the tubing substantially corresponds to the angle formed by a diagonal of the cross-section of the tubing wall and a flat face of the wall. Because of the very small angle involved, cutting forces in a direction normal to the tube walls are small compared to cutting forces parallel to the walls. Thus, even where the cutting blade is set at a level which will completely sever the flat walls of the tube, burrs will not be directed inwardly to the tube passage. Also, since the cutting blades attack the tube wall at an attitude corresponding to the diagonal of the wall to be severed, the effective cutting stroke of the blade is twice the width of the tube. This may be contrasted with the total travel of opposed circular cutters in partially severing the opposed walls of the tube wherein such travel is slightly greater than the width of the tube, and the total travel of the cutters in the Wohrstein et al. patent wherein the total travel of the cutters is slightly less than the wall thickness. Since the total work involved in severing or nearly severing the opposed walls of the tube is identical in all of the above situations and since the cutting stroke, according to the present invention, is maximized, the forces associated with the cutting operation are minimized, particularly those forces normal to the plane of the flat walls of the tube.
After the opposed walls are severed, the tube is completely parted by applying an axial force to the tubing to exceed the ultimate strength of the unsevered tube sides and the plurality of internal parallel walls which define the axially extending internal chambers. According to a preferred aspect of this invention, the force applied to the tubing is a high impact force along the longitudinal axis of the tubing.